1. Field of the Invention
This invention relates to a hydrostatic drive system with an adjustable pump in the delivery flow and at least one consuming device connected to the pump. More specifically associated with each consuming device are actuation means that specify the direction of movement and the speed of movement, and a control valve to control the direction of movement and the speed of movement, and wherein the delivery flow of the pump can be adjusted to the hydraulic fluid flow required by the actuated consuming devices.
2. Background Information
Similar hydrostatic drive systems for the control of a plurality of consuming devices are load-sensing drive systems in which the delivery flow of the pump can be adjusted to the hydraulic flow required by the consuming devices. In these systems, when the consuming device is actuated by the control valve, the load pressure of the consuming device is measured, transmitted to a pump regulator that controls the delivery volume of the pump and compared to the delivery pressure of the pump.
The speed of the consuming device is controlled by a control valve that, when actuation means are actuated, is deflected to correspond to the specified speed and direction of movement. The opening width of the control valve thereby forms a measurement throttle that determines the speed of movement of the consuming device. By measuring the load pressure of the consuming device and comparing it with the delivery pressure supplied by the pump at a pump regulator that controls the delivery volume of the pump, the pump thereby delivers a flow that corresponds to the hydraulic flow required by the consuming device. Thereby a delivery pressure is achieved that is higher than the load pressure of the consuming device by a defined control pressure that equals the spring bias on the pump regulator.
In the event of a simultaneous actuation of a plurality of consuming devices, the highest load pressure of the actuated consuming devices is transmitted to the pump regulator, as a result of which a delivery pressure is set that is above the highest load pressure by the control pressure.
For operation of the consuming devices independently of the load, associated with each control valve there is a pressure balance that forms an additional throttle point and keeps the pressure differential at the measurement throttle formed by the opening width of the control valve constant. The pressure balances that are associated with the control valves thereby prevent the consuming devices with the lower load pressures from being operated at an excessive speed of movement. The consuming devices are thereby operated independently of the load at the respective speed of movement set at the control valve.
With such a control valve, however, for the control of the consuming devices at the control valve and at the pressure balance associated with the control valve, there must be a pressure difference. The level of this pressure difference thereby determines the level of the control pressure set at the pump regulator. Because the delivery pressure supplied by the pump equals the sum of the load pressure and the control pressure, the level of the control pressure results in pressure losses and thus in energy losses in the hydrostatic drive system.
In addition, with such a control consisting of a pressure balance and control valve, only the hydraulic flow to the respective consuming device can be controlled. In the event of a change in the load direction of the load being applied to the consuming device, for example in the event of a change from a positive load to a negative load, there is an increase in the speed of movement of the consuming device. To prevent such an increase in the speed of movement in the event of a change in the direction of the load, corresponding valve means, for example braking valves, must be provided on the discharge side of the consuming device, which makes the construction more complicated and expensive.
DE 37 16 200 C2 describes a control and regulation device for a hydrostatic drive system with a pump and a plurality of consuming devices, in which the speed of movement of each consuming device is compared with a specified speed of movement in an electronic control, and the delivery of the pump is adjusted so that the set speed of movement of the consuming devices can be achieved with low energy losses. The measurement of the actual speed of movement is thereby measured by position or speed-of-rotation sensors on the consuming devices, or by flow meters, in particular rotating impeller wheels, by which the delivery flow of a consuming device can be measured. Position or speed-of-rotation sensors are expensive and complicated, however, and must also be located in the immediate vicinity of the consuming device. When such a drive system is used in a machine such as an excavator, for example, however, the consuming devices are exposed to environmental factors such as dirt, moisture, etc. The sensors must also be protected against these environmental influences. The electrical connections between the sensors that are located on the consuming devices and the electronic control must also be protected against mechanical damage. This environmental protection is both complicated and expensive. When flow meters are used, of course, the consuming devices can be operated independently of the load, but if the direction of the load being exerted on the consuming device changes, for example from a positive to a negative load, the consuming device cannot be operated independently of the load.
The object of the invention is therefore to make available a hydrostatic drive system of the type described above that makes it possible to operate the consuming devices independently of the load, and to operate the consuming devices independently of the direction of the load with low energy losses, thereby requiring little construction effort and expense.